This invention relates to DC-DC switching regulators, and more particularly relates to improved overcurrent sensing for such regulators, for high speed operation.
DC-DC switching regulators, or, converters, are circuits that use an inductor, a transformer, or a capacitor as an energy storage element to allow the transfer of energy from a switched element connected to its input to its output in discrete packets. One type of DC-DC switching regulator uses an inductor as an energy storage element supplying current to a capacitor used as a charge storage element at the output. The inductor and capacitor also serve as a filter for the output voltage. Feedback circuitry regulates the energy transfer from the switched element to the energy storage elements so as to maintain a relatively constant voltage across the charge storage element, within the load limits of the circuit.
DC-DC switching regulators can be configured to step up (boost) or step down (buck) the output voltage, or both, and can be configured to invert output voltage with respect to input voltage. A benefit of DC-DC switching regulators is their relatively high efficiency. In basic configurations, a so-called xe2x80x9cfreewheelingxe2x80x9d diode, such as a Schottky-type diode, is used as a rectifier to allow current to flow from the current energy storage element to the charge storage element during the discharge phase of a cycle when the switching element is turned off, but which is reverse-biased during the charge phase of a cycle when the switching element is turned on. A typical operating frequency is on the order of 500 KHz, although the frequency is quite variable, depending on design considerations.
When DC-DC switching regulators are used in low output voltage applications, the power dissipation induced by the freewheeling diode of basic designs can be excessive. To alleviate this problem, a switch is sometimes used in the place of the freewheeling diode as the rectifier, and the resulting regulator is said to be synchronous. The switch is typically a power metal oxide semiconductor field effect transistor (MOSFET) device. In synchronous switching regulators, the switch that regulates the pulses of energy to the energy storage element is frequently called the high side switch, while the switch replacing the freewheeling diode is frequently called the low side switch. Since they are typically both power FET devices, they are called the high side device, or transistor, and low side device, or transistor, respectively. They are driven by a high side drive and a low side drive, respectively. The low side drive is the inverse of high side drive, usually with a dead zone, or, dead band, at transitions to prevent brief moments when both switches would otherwise be on at the same time.
DC-DC switching regulators typically have protection circuits included in their design. One type of protection circuit senses the approach of excessive operating current that could damage components in the regulator. When such excessive current is detected, an overcurrent sense signal is generated, and used to enter a protective mode. A commonly used overcurrent sense/protect circuit provides cycle-by-cycle current limiting when an overcurrent condition is sensed.
To provide cycle-by-cycle current limiting, switching regulators must sense operating current during the portion of the switching cycle when the energy source is connected to the energy storage element. For example, in a voltage mode buck converter, it is common to use either the xe2x80x9conxe2x80x9d resistance of the high side device or a low value resistor in series with the high side device as a current sense element. The measured voltage across the resistance is used to compute the current. However, narrow pulse width switching regulators have an inherent problem sensing input current using the high side device or a resistor in series with it as the sense element. Parasitic ringing of the voltage at the switched node causes false or nuisance tripping of the overcurrent sense circuit, when the overcurrent voltage threshold is set at a normal level. On the other hand, if the overcurrent voltage threshold is set higher to avoid this, excessive or destructive current may flow before the overcurrent protection circuit is activated.
Another problem related to overcurrent sensing and protecting arises from the turn-on delays in power FETs. Because of such delays, a blanking time is frequently designed into overcurrent detection circuits, to ensure the high side device is actually on when the circuit monitors the voltage across it to determine if an overcurrent condition exists. If the device is not actually on, its impedance is very high and therefore a voltage is likely to exist across it that greatly exceeds the overcurrent voltage threshold. However, power FETs vary considerably in their turn-on delay times, and the resultant varying turn-on delays in switching regulators pose problems for setting the necessary blanking time for overcurrent detection. Propagation delays inside the controller only make the problem worse. As pulse widths get narrower, a fixed blanking time can be made to work reasonably well if adequate control of the switched node ringing is maintained, for example using snubber circuits and limiting the switching time with gate resistors in the main switch gate circuit. However, with very narrow pulse widths, such measures may be inadequate and the turn-on delays and propagation delays may be appreciable when compared to the nominal pulse width. This makes setting an effective generic overcurrent blanking time difficult.
It would therefore be desirable to have DC-DC switching converters operable with narrow drive pulse widths, for example in high frequency drive configurations, with improved overcurrent sensing. It would be desirable to have such converters in which the high side drive device can be used to sense voltage for overcurrent
In accordance with the present invention there is provided a DC-DC switching regulator, adapted to receive a pulsed signal. The regulator includes an inductor, and also includes a capacitor having one port connected to ground, and having a second port providing an output voltage of the DC-DC regulator. A driver is coupled to the inductor and adapted to drive pulses of current to the inductor when the pulsed signal is asserted. A rectifier is adapted to provide a path for the inductor to drive current to charge the first capacitor when the pulsed signal is not asserted. An overcurrent circuit is provided, adapted to sense a threshold current of the switching regulator corresponding to an overcurrent condition and to provide an overcurrent indication signal in response thereto. The overcurrent circuit includes a ringing compensation circuit adapted to control the overcurrent circuit threshold for providing the overcurrent indication signal from a first level to a subsequent second level less than the first level. The overcurrent circuit may also be provided with a delay circuit adapted to sense a predetermined enablement parameter, and in response to enable the overcurrent circuit.